Crystallization of seed globulins from legumes.
Seeds contain large quantities of proteins and are therefore main food sources. In the last century, protein extracts of legume seeds were dialysed against distilled water and in some cases small crystals of pure protein appeared. However, those crystals were generally of poor quality with respect to X-ray diffraction. Recently, the crystallization of some of them was improved and the structures of two 7S globulins, phaseolin from Phaseolus vulgaris and canavalin from Canavalia ensiformis, have been determined at 3.0 and 2.6 A resolution, respectively. Efforts to improve the quality of the phaseolin crystals resulted in three new crystal forms which will be discussed in this paper. The only high-resolution X-ray analysis of a seed globulin from legumes is that of narbonin, a 2S protein from Vicia narbonensis. The crystal structure at 1.8 A shows a very compact packing in layers of molecules. The intermolecular contacts include salt bridges and hydrophobic clusters that might facilitate both the aggregation of the molecules and their crystallization. Because the seed globulins appear in large quantities in the protein bodies of the seeds, efficient packing of the molecules similar to the crystal packing can be assumed.